Multihop packet radio networks (i.e., ad-hoc networks) extend packet switching technology into environments with mobile users. Such networks can be installed quickly in emergency situations, and are self-configurable. The medium access control (MAC) protocol that allows packet radios (or stations) to share a common broadcast channel is an essential component of a packet radio network.
There are two main classes of MAC protocols: contention-based protocols and contention-free protocols. Carrier sense multiple access (CSMA) protocols (see, e.g., L. Kleinrock & F. A. Tobagi, “Packet Switching in Radio Channels: Part I—Carrier Sense multiple Access Modes and their throughput-Delay Characteristics,” IEEE Trans. Comm., Vol. COM-23, No. 12, pp. 1400–1416 (1975)) are one the most popular examples of contention-based MAC protocols and have been used in a number of packet radio networks in the past, such as described in B. M. Leiner et al., eds. Proceedings of the IEEE, Vol. 75 (January 1987). These protocols attempt to prevent a station from transmitting simultaneously with other stations within its transmitting range by requiring each station to listen to the channel before transmitting.
The hardware characteristics of packet radios are such that a packet-radio cannot transmit and listen to the same channel simultaneously; therefore, collision detection (e.g., CSMA/CD, as described by R. M. Metcalfe & D. R. Boggs, “ETHERNET: Distributed Packet Switching for Local Computer Networks,” Communications of the ACM, Vol. 19, No. 7, pp. 395–403 (1976)) cannot be used in a single-channel packet radio network. Further, although the throughput of CSMA protocols is very good, as long as the multiple transmitters within range of the same receivers can sense one another's transmissions, “hidden terminal” problems degrade the performance of CSMA substantially. This is because carrier sensing cannot prevent collisions in that case. See, e.g., F. A. Tobagi & L. Kleinrock, “Packet Switching in Radio Channels: Part II—the Hidden Terminal Problem in Carrier Sense Multiple Access Modes and the Busy-Tone Solution,” IEEE Trans. Comm., Vol. 23, No. 12, pp. 1417–1433 (1975).
The busy tone multiple access (BTMA) protocol advanced by Tobagi and Kleinrock was the first proposal to combat the hidden-terminal problems of CSMA. BTMA is designed for station-based networks and divides the channel into a message channel and the busy-tone channel. The base station transmits a busy-tone signal on the busy-tone channel as long as it senses carrier on the data channel. Because the base station is in line of sight of all terminals, each terminal can sense the busy-tone channel to determine the state of the data channel. The limitations of BTMA are the use of a separate channel to convey the state of the data channel, the need for the receiver to transmit the busy tone while detecting carrier in the data channel and the difficulty of detecting the busy-tone signal in a narrow-band channel.
A receiver initiated busy-tone multiple access protocol for packet-radio networks has also been proposed. C. Wu & V. O. K. Li, “Receiver-Initiated Busy-Tone Multiple Access in Packet Radio Networks,” ACM SIGCOMM 87 Workshop: Frontiers in Computer Communications Technology, Aug. 11–13, 1987. In this scheme, the sender transmits a request-to-send (RTS) to the receiver, before sending a data packet. When the receiver obtains a correct RTS, it transmits a busy tone in a separate channel to alert other sources nearby that they should backoff. The correct source is always notified that it can proceed with transmission of the data packet. One limitation of this scheme is that it still requires a separate busy-tone channel and full-duplex operation at the receiver, thereby making it impractical for packet radio networks.
One of the first protocols for wireless networks based on a handshake between sender and receiver was SRMA (split-channel reservation multiple access). F. A. Tobagi & L. Kleinrock, “Packet Switching in Radio Channels: Part III—Polling and (Dynamic) Split-Channel Reservation Multiple Access,”IEEE Trans. Comm., Vol. COM-24, No. 8, pp. 832–845 (1976). According to SRMA, the sender of a packet uses ALOHA or CSMA to decide when to send a clear-to-send (CTS) if it receives the RTS correctly. The CTS tells the sender when to transmit its data packet. Although SRMA was proposed with one or two control channels for the RTS/CTS exchange, the same scheme applies for a single channel.
Since the time SRMA was first proposed, several other MAC protocols have been proposed for either single-channel wireless networks or wireline local area networks that are based on similar RTS-CTS exchanges, or based on RTSs followed by pauses. See, e.g., V. Bharghavan et al., “MACAW: A Medium Access Protocol for Wireless LANs,” Proc. ACM SIGCOMM '94, pp. 212–25, Aug. 31–Sep. 2, 1994; V. Bharghavan, “Access, Addressing and Security in Wireless Packet Networks, PhD Thesis, University of California, Berkeley, Computer science Dept. (1995); A. Colvin, “CSMA with Collision Aviodance,” Computer Comm., Vol. 6, No. 5, pp. 227–235 (1983); W. F. Lo & H. T. Mouftah, “Carrier Sense Multiple Access with Collision Detection for Radio Channels, ” IEEE 13th Int'l Comm. And Energy Conf., pp. 244–247 (1984); R. Rom, “Collision Detection in Radio Channels” pp. 235–49 (1986); and G. S. Sidhu et al., “Inside Apple Talk,” 2d ed. (1990). In addition, Karn proposed a protocol called MACA (multiple access collision avoidance) to address the problems of hidden terminals in single-channel SRMA using ALOHA for the transmission of RTSs. P. Karn, “MACA—A New Channel Access Method for Packet Radio,” ARRL/CRRL Amateur Radio 9th Computer Networking Conference, pp. 134–140 (1990). This protocol attempts to detect collisions at the receiver by means of the RTS-CTS exchange without carrier sensing. A committee of the Institute of Electrical and Electronic Engineers (IEEE) overseeing the 802.11 specification for computer networking has proposed a MAC protocol for wireless LANs that includes a transmission mode based on an RTS-CTS handshake (DFWMAC). K. C. Chen, “Medium Access Control of Wireless LANs for Mobile Computing,” IEEE Network, Vol. 8, No. 5, pp. 50–63 (1994); P802.11—Unapproved Draft: Wireless LAN Medium Access Control (MAC) and Physical Specifications, IEEE (January 1996).
Fullmer and Garcia-Luna-Aceves introduced a new variation on MAC protocols based on RTS-CTS exchanges that is particularly attractive for ad-hoc networks. C. L. Fullmer & J. J. Garcia-Luna-Aceves, “Solutions to Hidden Terminal Problems in Wireless Networks,” Proc. ACM SIGCOMM '97, Sep. 14–18 (1997). This protocol is FAMA-NCS (floor acquisition multiple access with non-persistent carrier sensing). The objective of FAMA-NCS is for a station that has data to send to acquire control of the channel in the vicinity of the receiver (which is termed “the floor”) before sending any data packet, and to ensure that no data packet collides with any other packet at the receiver.
The main problem with contention-based MAC protocols such as those discussed above is that they do not provide delay access guarantees. This shortcoming renders contention-based protocols inefficient for such applications as void transfer over wireless networks. Collision-free protocols can provide channel access delay guarantees; however, very few protocols have been designed to operate in multihop wireless networks.
Another protocol family used in networks is the time division multiple access (TDMA) protocol family. Here, time is divided into frames consisting of time slots. Time slots are allocated to specific nodes or a centralized station is used to allocate the time slots. The limitations of TDMA stem from the fixed assignment of time slots to nodes, which is slow to adapt to network changes and makes inefficient use of the channel if nodes are bursty sources of traffic (such as is the case in ad-hoc environments), and the use of centralized assignments. A number of protocols have been proposed in the recent past to provide dynamic time-slot allocation without requiring central base stations. These protocols can be classified as topology-independent and topology-dependent time scheduling protocols.
Shepard, Chlamtac, and Ju and Li have proposed topology-independent time-scheduling protocols. T. Shepard, “A Channel Access Scheme for Large Dense Packet Radio Networks,” SIGCOMM '96 Conference Proc. (1996); I. Chlamatac et al., “Time-Spread Multiple-Access (TSMA) Protocols for Multihop Mobile Radio Networks,” IEEE/ACM Transactions on Networking, Vol. 5, No. 6 (December 1997); Ji-Her Ju & Victor O. K. Li, “An Optical Topology-Transparent Scheduling Method in Multihop Packet radio Networks,” IEEE/ACM Transactions on Networking Vol. 6, No. 3 (June 1998). In these protocols, nodes are pre-assigned (by means of their nodal IDs, for example) or adopt a transmission schedule that they publish, and such a schedule specifies the times when a node transmits and receives. The protocols guarantee or provide a high likelihood that at least one transmission time in a node's schedule does not conflict with any node one or two hops away.
In the Chlamtac and Ju approaches, nodes are unable to determine which transmissions will succeed, complicating the job of higher layer (e.g., link-layer) protocols. These approaches also require values for the total number of nodes in the network and maximum number of neighbors for each node, as input parameters to the algorithm, thus making them design for the worst case conditions (and thus, resulting in inefficiencies if the network is not as dense as expected), or being sensitive to actual network conditions (if the network is larger or more dense than expected).
Shepard's approach avoids collisions by assuming nodes are synchronized with their neighbors, have knowledge of their neighbors' schedules, and are able to receive from multiple transmitting neighbors simultaneously. This final assumption requires fairly sophisticated radio hardware.
Recently, Zhu and Corson (C. Zhu & M. S. Corson, “A Five-Phase Reservation Protocol (FPRP) for Mobile Ad Hoc Networks,” Proc. IEEE INFOCOM '98) and Tang and Garcia-Luna-Aceves (Z. Tang & J. J. Garcia-Luna-Aceves, “Hop-Reservation Multiple Access (HRMA) for Multichannel Packet Radio Networks,” Proc. IEEE IC3N '98: Seventh Int'l. Conf. On Computer Communications and Networks, Oct. 12–15, 1998; J. J. Garcia-Luna-Aceves, “SPARROW/WINGS Technologies,” DARPA/SPAWAR Meeting, SPAWAR, San Diego, Nov. 18, 1998) have developed topology-dependent scheduling protocols, such that a node acquires a transmission schedule that allows the node to transmit without interfering with nodes one and two hops away from itself, and such that channel reuse is increased as the number of neighbors per node decreases. These protocols require nodes to contend in order to reserve collision-free time slots.
Other TDMA approaches that require an initial, topology-independent schedule, followed by communication among the network nodes to negotiate a final schedule include the following. Chlamtac proposed an algorithm based on a repeating link schedule that can adapt to traffic demands after some number of iterations of the algorithm. The algorithm starts with a “single-slot-per-link” schedule, such as provided by assigning each node a transmission slot according to its node ID. At each iteration, schedule information and a scheduling “token” are routed up and down a routing tree (established by means of pre-existing algorithms), to assign additional slots to nodes or links according to their degree of unmet traffic demands. I. Chlamtac, “Fair Algorithms for Maximal Link Activation in Multihop radio Networks,” IEEE Transactions on Communications, Vol. COM-35, No. 7 (1987).
Ephremides and Truong proposed a similar algorithm in which each node is initially assigned a slot corresponding to its node ID, and then each node uses their assignment to pass “skeleton” schedules to their neighbors. During the next two frames (two iterations of communicating schedules), and in accordance with fixed node priorities, nodes are able to grab available slots until all available slots are taken (i.e., no more slots can be assigned without causing collisions. Because of the need for schedules that are relatively fixed, requiring a few iterations to converge, and of scheduling-frame size equal to the maximum size of the network, these approaches have limited scalability and robustness to mobility or other dynamics. A. Ephremides & T. Truong, “Scheduling Broadcasts in Multihop Radio Networks,” IEEE Transactions on Communications, Vol. COM-38, No. 4 (1990).
The approach proposed by Young also requires initial assignment of one slot per node, and then negotiation of scheduling packets for assignment of the other slots. However, the initially assigned slot is limited to the first slot in each “frame.” Thus, each node's assigned slot occurs every N frames, where N is the maximum network size. Because of this, the approach is not scalable. Also, because a node needs to wait up to N frames before a neighbor confirms a proposed schedule addition, the approach is relatively slow adapting to dynamic traffic conditions. C. David Young, “USAP: a unifying dynamic distributed multichannel TDMA slot assignment protocol”, MILCOM '96 Conf. Proc., vol. 1, pp. 235–239 (October 1996).